1. Field of the Invention
This invention relates to nonvolatile memory elements, and more particularly, to methods for forming resistive switching memory elements used in nonvolatile memory devices.
2. Description of the Related Art
Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments.
Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals.
As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly smaller dimensions. However, as device dimensions shrink, scaling issues are posing challenges for traditional nonvolatile memory technology. This has led to the investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory.
Resistive switching nonvolatile memory is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of suitable voltages or currents. Voltage pulses are typically used to switch the memory element from one resistance state to the other. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell.
Resistive switching based on transition metal oxide switching elements formed of metal oxide (MO) films has been demonstrated. Although metal oxide (MO) films such as these exhibit bistability, the resistance of these films and/or the ratio of the high-to-low resistance states is (are) often insufficient to be of use within a practical nonvolatile memory device. For instance, the resistance states of the metal oxide film should preferably be significant as compared to that of the system (e.g., the memory device and associated circuitry) so that any change in the resistance state change is perceptible. Since the variation in the difference between the high and low resistive states is related to the resistance of the resistive switching layer, variations in the magnitude of the difference between states and differences between the states throughout the life of the device is often not desirable. Moreover, in cases where multiple formed memory devices are interconnected to each other and to other circuit elements, it is desirable to minimize the device performance variation between one device to the next to assure that the performance of the formed circuit performs in a desirable manner.
Thus, in a conventional resistive switching memory device manufacturing process, additional post fabrication processing steps are commonly required to assure that the resistive state change or resistive switching properties of the memory device do not vary with time and have a sufficiently perceptible difference between the bi-stable states. Typically, these additional processing steps include an “electrical forming” process that requires the application of a forming voltage at least once across the memory device to “burn-in” the device.
However, it is desirable to minimize and/or eliminate the need for these post processing steps to reduce the added complexity of the manufacturing process created by the addition of the post processing steps, reduce the length of the memory device manufacturing process and reduce the possibility of physically harming the material found in the resistance switching layer by the application of the often high electrical currents that are required during the “electrical forming” type post processing steps.
Therefore, there is a need for a resistive switching memory device that does not require the use of these post processing steps, such as the “electrical forming” type post processing step, and has consistent and improved resistive switching characteristics.